1. Field of Invention
The present invention relates to a method for dissolving poly(paraphenylene-terephthalamide) (hereinafter “PPTA”) or copolymers in sulfuric acid using a twin screw extruder.
2. Description of Related Art
Traditionally, methods for dissolving PPTA include dissolving PPTA in liquid sulfuric acid in a reactor or kneader and degassing the formed solution. This method takes about 4 hours at a temperature of 85° C., which causes degradation of the polymer or copolymer. Alternatively, mixing PPTA with (frozen) sulfuric acid ice is performed at low temperature (below 25° C.), followed by melting and degassing the produced solid solution. The method takes about 6 hours and has a large residence time distribution. This process is limited to mixtures having a PPTA-content between 17.5 and 19.8 wt. %. Below 17.5 wt. % PPTA in sulfuric acid, a substantial amount of off-spec material is formed. Further, copolymers cannot be dissolved or are only to dissolve with difficulties because the formed solid solutions become too sticky and/or formed into big lumps that cannot easily be processed further. These known methods thus have inherent disadvantages.
Twin screw extruders are typically used to mix and extrude polymers. For instance, in NL 8500429, a polyethylene mixture was obtained by using a twin screw extruder. The mixture is suitable for making tapes, films, tubes, and the like. In other publications, for instance in EP 821708, a twin screw extruder is described for use in polymerizing an aliphatic diamine and an aliphatic dicarboxylic acid to obtain a non-aromatic polyamide, such as nylon. Use of polymers other than polyethylene or polyamide, or methods for making a spin dope, were not disclosed.